1. Field of the Invention
The present invention relates to a device for generating an electrical control signal according to a position of a light beam in relation to a border line or separating line extending transverse to the propagation direction of the light beam and, more particularly, to a device for generating an electrical control signal dependent on the position of the light beam in relation to the separating line which has two photodetectors, one which one is arranged to detect light falling on one side of the separating line and the other is arranged to detect light falling on the other side of the separating line.
2. Prior Art
A device for position determination of a light beam is required in a receiving unit, when information is transmitted between a transmitting unit and the receiving unit using the light beam. When the transmitting unit and/or receiving unit is movable, the beam direction must be continuously monitored during the transmission. The alignment of the beam must be performed with very slight angular tolerances, especially during transmissions over very large distances, such as between satellites.
Usually telescopes with lenses or mirror objectives are used for bridging very large distances. In order to transmit or receive a collimated light beam, the transmitting or receiving module must be mounted at the focal point of an objective. The image point of the received light beam moves in the focal plane of the objective during angular adjustment of the receiving telescope. The lateral spacing of the image point from the optical axis of the telescope is given by the product of the focal distance of the objective and the angular displacement between the beam propagation direction and the telescope optical axis. From the position of the image point the alignment of the telescope is determined and a control signal is generated for correction of the beam direction at the receiving device and/or transmitting device.
According to the state of the art there are several possibilities to determine the position of the light spot. For example the light spot can be received by a four quadrant diode, which detects the beam position by comparing the signals from the four quadrants. The beam alignment for coherent superimposed reception is described in European Patent Application EP-A-0831604. Light from a local oscillator is superimposed on the received signal and the overlapping signals are divided by means of beam splitter and sent along two light paths with an equal optical path length. Both beam parts are directed by separate optics comprising mirrors and lens onto two detection devices each comprising respective separate detector halves.
In the case of an ideal beam alignment both beam parts fall on the respective separating lines between the respective detector halves. A position deviation signal for the respective beam parts from their ideal or set positions is thus obtained by comparing the signal strengths from the detector halves of the respective detectors. This signal can be used for control. The separating lines between the detector halves of the respective detectors are displaced about 90.degree. relative to each other, so that a first control signal from one pair of the detector halves can be used for correcting the X-axis deviation, while the other or second control signal from the other pair of detector halves can be used for correcting the Y-axis deviation.
Because of the size of the optical beam guiding elements for both beam parts both detector arrangements with the separate detector halves must be spaced several millimeters from each other. In order to obtain the required high accuracy for the alignment detection, the positions of both separating lines between the detector halves of the both detector arrangements relative to each other and to the optical beam guiding elements for the beam parts must have a positional accuracy of about 1 micrometer. This accuracy must be achieved at a spacing or distance of several millimeters to a few centimeters because of the size of the optical beam guiding elements.
In the known method of satisfying these requirements both detector arrangements are mounted on a common substrate. Since the detectors also operate to receive the transmitted data signals as well as produce the control signal, only small surface area photodiodes can be used because of the high bit rate of the data signal in the vicinity of up to 10 Gbit/s. Because of that aspect of the detectors the requirements for the positioning accuracy of the detector arrangements are further sharpened or increased.